The invention relates generally for optical distance measurement.
Optical distance measuring devices per se have been known for a long time. These devices emit a modulated light beam, which is aimed at a desired target surface, whose distance from the device is to be ascertained. The light reflected from or scattered by the target face aimed at is in part detected again by the device and used to ascertain the distance sought. A distinction is made between so-called phase measuring methods and pure transit time methods for determining the distance sought. In the transit time measuring method, for instance, a light pulse of the shortest possible duration is emitted by the device, and its transit time to the target and back again into the measuring device is ascertained. From this, because the value of the speed of light is known, the distance from the device to the target can be calculated.
In the phase measuring methods, the variation in phase with the travel distance is utilized to determine the distance. Via the phase displacement of the returning light, compared to the light emitted, the route traveled by the light and thus the spacing from the target object can be determined.
The range of application of such distance measuring devices includes differences in the range from a few millimeters up to several hundred millimeters.
Depending on the travel distances to be measured and the capability of the target object to return a beam, different demands are made of the light source, the quality of the measurement beam, and the detector.
At relatively short distancesxe2x80x94in the range up to several metersxe2x80x94from a target that furthermore has good retro-reflectivity, a well-collimated light beam of limited diffraction will advantageously be used, in order to obtain good resolution and a correspondingly strong returning signal.
At great distances, the problem is meeting the desired target with a fine light beam, such as a focused, limited-diffraction laser beam, so that for this type of application, it is advantageous to use a measuring beam with a larger diameter.
The optical distance measuring devices known from the prior art can fundamentally be divided into two categories, depending on the disposition of transmission conduits and reception conduits. First, there are apparatuses in which the transmission conduit is disposed next to the reception conduit at a certain distance from it, so that the respective optical axes extend parallel to one another. Second, monoaxial measuring devices exist in which the reception conduit extends coaxially with the transmission conduit.
From German Patent DE 198 40 049 C2, a monoaxial device for optical distance measurement is known for geodetic, construction and industrial uses, with which both highly retro-reflective and poorly retro-reflective target objects can be measured. This apparatus furthermore has high resolution, even for poorly retro-reflective objects.
The apparatus of DE 198 40 049 C2 has a transmitter unit, with one or two optical radiation sources for generating two separable beams. One beam is limited in its diffraction and includes light in the visible wavelength range, while conversely the other beam is divergent and is in the visible or infrared wavelength range.
A disadvantage of the apparatus of DE 198 40 049 C2 is the complex generation of the various measuring beams. Depending on the type of embodiment, the device uses two separate laser diodes, or dual-wavelength lasers, to generate the two measuring beams. The different beam divergences are generated by complicated telescope lens elements in the two beam paths of this apparatus. The apparatus of DE 198 40 049 C2 furthermore requires selection means for detecting the different, coaxially extending types of radiation and beams. The selection means mentioned in DE 198 40 049 C2, such as optical filters, controllable frequency doublers, and Q switchers are complicated and expensive optical components that make the measuring system quite complex, inconvenient to handle, and expensive.
Biaxial measuring systems, by comparison, have the advantage of not requiring complicated beam splitting, so that optical crosstalk from the transmission conduit directly into the reception conduit can be better suppressed.
On the other hand, in biaxial distance measuring devices there is the disadvantage that for the range of short measurement distances, parallax can cause detection problems. The projection of the target object onto the detector surface, which for great target distances is still located unequivocally on the detector, increasingly migrates away from the optical axis of the receiving branch as the measurement distance becomes shorter, leading to a variation in the beam cross section in the plane of the detector.
The apparatus of the invention for optical distance measurement, has the advantage over the prior art that the beam generated in the distance meter can be adapted to the various measurement tasks (that is, different target objects or target distances that have to be measured) with merely a single optical means. In particular, the divergence and the direction of the measuring beam can be adapted to the particular measurement requirement with this single optical means.
Two separate, different beam paths for furnishing beams of different divergence are unnecessary in the apparatus of the invention. This advantageously makes a compact, handheld measuring device possible, for instance.
The different beam divergences of the measuring beam can be achieved by simply varying the relative disposition of the light source and an optical means.
In a first advantageous embodiment of the distance measuring device of the invention, the optical means used is a lens or lens combination that serves as a lens element and is placed for instance in the beam path of the transmitter unit of the measuring device. Other optical means can also be imagined and will be described later.
Advantageously, the apparatus of the invention for optical distance measurement can be realized by using a laser as the light source. Various kinds of miniaturized lasers that can be built into such measuring devices are presently available. Because of their small size and high power, semiconductor lasers are especially advantageous for the purpose; by now, they are available with suitable quality even for the visible part of the spectrum of electromagnetic waves.
To generate a collimated, limited-diffraction measuring beam, the light source is placed in the focus of the lens element on the object. The divergent beam emerging from the light source is collimated after passing through the lens element and forms a largely parallel measuring beam, which can for instance be used for the distance measurement with high resolution.
If a measuring beam of increased divergence is required, as a result of the need to measure relatively great measuring distances, then in the apparatus of the invention, the lens element lens can be moved in the direction of the optical axis of the measurement signal, in order to vary and adjust the desired divergence of the measuring beam.
Thus in the apparatus of the invention, it is possible for instance first to use a divergent beam, for aiming the device at a target object located far away, and after the measuring beam strikes the target object, the measuring beam can be recollimated with limited diffraction, in order to achieve good resolution and a clearly measurable returning signal in the measuring device.
To vary the beam divergence, the lens element lens is adjusted in its position by way of various actuators that are controlled by a closed-loop control mechanism.
In another embodiment of the apparatus of the invention, the position of the light source can be varied via corresponding actuators and an associated closed-loop control circuit relative to what is then optionally a fixed collimating lens element.
The control of the actuators that are capable of varying the relative disposition of the lens element and the light source is done manually in one embodiment of the device, in such a way that the user first widens the measuring beam, by pressing on a button on the control element of the device, for instance, and aims it at the target object, until the device shows him a returning signal. Next, the user can adjust the desired degree of collimation of the measuring radiation, in order to obtain increased resolution, for instance, or a suitably strengthened received signal. Point-precise measurement is thus possible, by reducing the size of the measurement spot, without the disadvantages that a small measurement spot represents for aiming at a target object that is for instance located far away.
In a further advantageous embodiment of the measuring device of the invention, the triggering of the actuators for the optical means, which means can for instance be a lens element, is taken on by a self-controlling closed-loop control circuit. Sensors at or in the measuring beam detect the beam diameter or the divergence of the beam and send this information on to an automatic closed-loop control mechanism. This closed-loop control mechanism then controls the actuators as desired, which assure a displacement of the lens element and/or of the light source as well. If the desired or required beam divergence is detected by the sensors, then the further displacement of the optical element is stopped.
Besides the divergence of the measuring beam, with the apparatus of the invention the position of the light beam can also be varied. By displacing the lens element perpendicular to the optical axis, the measuring beam can be deflected, for instance so that a certain angular range can be swept by a collimated beam of light.
In an advantageous embodiment of the apparatus of the invention, a xe2x80x9cscanning modexe2x80x9d is therefore provided, which makes it possible, by suitably fast displacement of the lens element lens, for the measuring beam to be deflected horizontally and/or vertically over an angular range. This embodiment of the apparatus of the invention advantageously makes it possible to locate target objects that are small or far away. By means of an automated closed-loop control circuit, of the kind for instance described above, it can be attained that the scanning mode is stopped once the receiver unit of the measuring device detects a returning signal. The closed-loop control mechanism can then set up the lens element in such a way that a maximum returning signal, for instance, is measured.
In another advantageous feature of the apparatus of the invention, a suitable optical means is located in the receiving branch of the measuring device. Typically, in devices of the prior art, there is a collimating lens in the receiving branch of the distance measuring device. This lens collects the returning light and focuses it onto a detector to enhance the power density. To that end, this collimating lens is located at the distance of its focal length in front of the detector, in order to focus light from a great distance exactly onto the detector.
If as the optical means of the invention a lens element that can be adjusted as described relative to the detector and thus also to the light source is placed in the receiving beam path, then it is possible to correct the focusing of the returning beam. In particular, with this embodiment of the apparatus of the invention, the parallax problem of biaxial measuring devices is overcome.
In biaxial measuring devices, because of a spatial separation, there are two separate, usually parallel optical axes for the transmitted signal and the received beam. Besides the dependency, already described above, of the focusingxe2x80x94and thus also the of size of the focus on the detectorxe2x80x94on the distance of the target object from the measuring device, a lateral shift in the returning beam relative to the optical axis of the receiving branch also occurs. This lateral shift can become so great that, for very short distances from the target object, the returning signal no longer strikes the detector, or strikes it only inadequately.
By means of a lateral shift of the optics in the receiving branch, this parallax-caused beam offset can be compensated for, and the measuring device of the invention can for instance be adjusted to a detection maximum for the returning radiation.
The variation in the position of the lens can advantageously be achieved once again via an automatic closed-loop control circuit, which by way of sensors determines the signal intensity of the detector, for instance, and triggers the actuators in such a way that for instance a maximum for the detected signal results.